1. Field of the Invention
The invention relates to radio frequency stabilization and, more particularly, frequency drift compensation in a digital radio system.
2. History of the Prior Art
A fundamental concept underlying radio communications systems is that transmission and reception must occur at specific operating frequencies and that such frequencies must be stable over a period of time. Most systems achieve frequency stability by including a crystal oscillator which generates a very precise frequency which is then used to produce the various high frequency signals employed in the different components of the radio. Crystals are, however, subject to slight variations in their resonant frequency of oscillation due to various environmental conditions and other factors. At resonant frequencies in the megahertz range, even small percentage variations result in sufficiently large changes in operating frequency to affect the operation of the radio. For example, a high quality standard reference crystal oscillator may have a frequency stability on the order of .+-.10 PPM. Thus, the different factors which cause variations in the frequency of oscillation of a reference crystal must be taken into account in radio design.
The three principle factors which cause variations in the frequency of oscillation of a crystal are temperature, aging, and the voltage applied to the crystal itself. To provide frequency stabilization in larger radio systems, such as those used in the base stations of cellular radio systems, the reference crystal is frequently mounted within an oven which is held at a selected temperature to a high degree of accuracy to reduce variations in the crystal's output frequency due to temperature changes. An oven reference crystal oscillator can maintain a frequency stability of .+-.0.1 PPM. Other more compact radio circuits, such as cellular mobile stations, cannot include temperature stabilizing ovens because of size and power consumption and must rely on various techniques to compensate for frequency variations in the crystal due to temperature.
Certain radio systems, both transmitters and receivers, include a relatively wide information channel and therefore slight variations in the tuning frequencies in the radios can be tolerated and still work properly. However, in radios having relatively narrow channels, the oscillator frequencies must be very precise in order to insure that the radio transmitter and receiver remain tuned to the desired channel rather than an adjacent channel at a slightly different frequency.
In digital radio systems forming part of a radio network, each of the radio channels are frequently very narrow, e.g., 12.5 KH.sub.z, and this requires good frequency stability in the mobile stations of the system. Further, such systems operate at very high data speeds, e.g., on the order of 8K bits/second. In order to achieve such high data rates, a modulation technique such as low pass filtered FSK (modified GMSK) is used which requires a high level of frequency stability in the carrier frequency of the transmitter. For example, frequency stability on the order of .+-.1.5 PPM (.+-.1.35 KH.sub.z at 900 MH.sub.z carrier) is often specified to obtain the desired accuracy in data transmission. A digital radio system similar to the type in which the present invention is used is disclosed in U.S. patent application Ser. No. 07/560,784, filed Jul. 31, 1990, entitled "A Method of Adopting a Mobile Radio Communications System To Traffic and Performance Requirements" and which is assigned to the assignee of the present invention and hereby incorporated by reference herein.
One approach to the periodic measurement and adjustment of the operating frequency within the mobile station of a cellular communication system is to receive the carrier signal from the base station and use that signal as a standard against which the local oscillator within the mobile station is compared and adjusted. Since the base station generally includes a frequency standard having an oven temperature stabilized crystal oscillator, its frequency is relatively stable and suitable for use as a standard in frequency stability compensation. For example, in U.S. Pat. No. 4,921,467 to Lax, a signal transmitted to a receiver is used to tune and adjust the frequency of the local oscillator within the receiver. In the Lax patent, frequency stability is obtained in a radio receiver by compensating for variations in temperature and other factors which may affect the frequency of the crystal. In this technique, a radio is initially calibrated during manufacture to include a stored table of voltage compensation values as a function of temperature for the voltage controlled local oscillator so that at a given temperature, a correct value of compensation voltage may be applied to obtain the proper frequency of oscillation. In addition, a signal is received from a transmitter and processed with the output of the voltage controlled local oscillator so as to generate an error signal if there is any difference between their respective frequencies. A correction voltage is produced, stored and used to correct the output of the local oscillator for variations in the frequency of the crystal from that of the transmitter.
In radio receiver frequency stabilization systems such as that taught by the Lax patent, crystal frequency compensation due to aging is relatively straight forward because of the availability of a continuous signal from a transmitter which can be used as a comparison standard along with feedback to properly correct the local oscillator frequency. However, in digital packet radio systems, numerous additional problems are present which do not allow the straightforward use of such systems.
In digital packet radio communications, there are often a plurality of different systems working simultaneously on the same frequency channels within a radio network. That is, a mobile station must be careful to insure that the signal that it is receiving and using as a frequency standard come from its own base station and not from some other base station broadcasting on the same channel. In addition, radio frequency interference and other spurious output signals may produce RF signals on the same frequency and a mobile must not mistake any of these signals for that of its base station when seeking to measure the stability of its reference oscillator frequency.
Another aspect of digital packet radio systems which makes it difficult to use a signal transmitted from a base station to measure and frequency stabilize the reference oscillator of the mobile station is that data signals are broadcast in burst mode. That is, the transmissions are all very short bursts of RF energy followed by the absence of RF signals in the spacing between bursts. This means that a mobile must be capable of making frequency measurements of the signal transmitted by the base station very quickly. It must also measure the true mean frequency of the transmitted signal even though the carrier signal is usually modulated with digital data.
The system of the present invention overcomes these and other disadvantages of the prior art and enables the mobile station of a digital packet radio system to periodically measure the frequency of the signal being transmitted from its base station and use that signal to adjust the output of its own crystal-controlled reference oscillator. This ensures that both the transmitting circuitry and receiving circuitry of the mobile station is properly frequency stabilized for each of the potential variable parameters which could cause the reference frequency oscillator to be operating at less than a very high degree of frequency stability.